sys-libs/glibc: disable stripping for cross-glibc

sys-libs/glibc: disable stripping for cross-glibc

commit e14229b10b513a164f8379ff14cc8c644c071f27 dropped
special stripping and relied on standard ${CHOST}-strip.

Unfortunately one special case wa missing:
cross-*/glibc ebuilds use ${CTARGET} as a target and are
installed into the host. That causes ${CHOST}-strip to be
called on ${CTARGET} binaries. If multitarget is not enabled
strip either breaks such bianries or makes them untouched.

Disable tripping until we migrated off off cross-*/glibc ebuilds.

Reported-by:  Johannes Geiss
Closes: https://bugs.gentoo.org/697428
Package-Manager: Portage-2.3.76, Repoman-2.3.17
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>